Since great frictional resistance is applied to a front wheel serving as a steering wheel, a power steering system provides assistant steering force to allow a driver to smoothly manipulate a steering handle. The assistant steering force is determined by measuring torque applied to a torsion bar when the steering handle is steered.
In order to measure torque of the steering wheel, various torque measuring devices have been developed and used. Among them, a torque measuring device employing a magnetic field has been mainly used since it has advantages in terms of a number of parts and price.
In the torque measuring device employing the magnetic field, a magnet is coupled to a steering handle that is an input shaft, one side of a stator holder fixing a stator is coupled to the input shaft through the torsion bar, and the other side of the stator holder is coupled to an output shaft of a front wheel.
Accordingly, when rotating the steering handle by applying external force to the steering handle, torque is measured by detecting a magnetic field according to a difference between a rotational angle of the magnet coupled to the input shaft and a rotational angle of the stator coupled to the stator holder to rotate together with the output shaft. The assistant steering force is determined based on the measured torque and a motor is driven according to the assistant steering force.